Before the js/eval stuff, it uses rpc calls dispatched via a defmethod. That’s only used for distributing ports to construct the mesh. Once the mesh is constructed, it then bootstraps the in macro that allows for js/evaling over the wire, and everything else after than is just invocation through the in macro.
Edit: Oh, and in Safari all spawn calls are proxied to the screen because it doesn’t yet support nested workers. In Chrome and Firefox :root handles all spawns.